[asterisk-dev] clearmode again (RFC 4040)

[Klaus Darilion]

Am 25.05.2010 18:17, schrieb Tilghman Lesher:
> On Tuesday 25 May 2010 09:29:04 Klaus Darilion wrote:
>> Some time ago we had lots of discussion about adding a clearmode
>> "pseudo-codec" to Asterisk.
>>
>> As (citation from RFC 4040)
>>
>>   >     "Clearmode" is a basic feature of VoIP Media Gateways.
>>
>> I wonder if now the time has come for Asterisk to be open for such an
>> feature?
>
> I don't think we're necessarily opposed.  The issue is that I'm not sure how
> useful such a provision would necessarily be.  While I'm clear about the
> usecase, I'm not clear about the practicality of such.  The RFC specifically
> says that it makes no provision for the problem of lost packets, and on that
> count, it falls short.  ISDN circuits are provisioned with the possibility of
> data corruption, but not with the possibility of losing 800 frames (10ms) all
> at once.
>
> It's much better to standardize how to correctly interpret a conversation on
> the ISDN level, packetize the message and establish a mechanism to ensuring
> that all packets arrive, then reassemble the digital signal on the other end,
> much the way T.38 works.
>
> Do you have a real need for this, or is this just a whiz-bang feature?

I ask because I just had a request from a customer who needs it for 
their customers.

I agree it is a bad solution to tunnel digital data, which should be 
transmitted reliabe, over VoIP. But there is plenty of ISDN legacy 
around which need to work also without a physical ISDN line. If it works 
or not will probably depend on the quality of the IP connectivity and of 
if the application has some error recovery on application layer (for 
example a video telephony application which uses digital ISDN channels 
can be capable of re-synchronization to the data stream after packet loss).

I also think it is a legitimate solution to support generic digital 
connections in a service provider infrastructure (LAN), e.g:

digital ISDN-->Asterisk1<--sip+clearcode-->Asterisk2<--ISDN digital

Regarding implemenation: I think it is rather easy to add the clearmode 
codec to core and chan_sip - just define a new audio codec like for 
example G729 pass-through.

Complex is only the handling in dahdi, libpri and chan_dahdi as dahdi 
itself might do transcoding.

regards
Klaus